Darrell Elton

Microcomputer-based instrumentation for multi-frequency Fourier transform alternating current (admittance and impedance) voltammetry

Microcomputer-based instrumentation has been developed which enables high quality Fourier transform alternating current admittance and impedance voltammograms to be obtained over a wide range of frequencies and d.c. potentials. A carefully chosen phase-optimised small amplitude alternating potential waveform containing sets of frequencies which avoid harmonic or intermodular interference are periodically superimposed onto a staircase d.c. ramped voltage. Considerable versatility is available in the choise of the excitation waveform and the d.c. ramp. Frequencies in the range of about 50 to 50 000 Hz may be applied at many potentials in any given experiment. Data are reported at both macro- and microelectrodes to illustrate the excellent signal-to-noise ratios that can be obtained under conditions commonly encountered in mechanistic or analytical applications of voltammetric techniques. In the analytical context, linear plots of peak height vs. concentration were obtained when using the admittance mode for both reduction of [Fe(CN)6)]3− at a carbon fibre microelectrode and reduction of Cd2+ at a hanging mercury drop electrode. Many mechanistic and analytical applications presently associated with single frequency linear sweep and cyclic a.c. voltammetry are expected to be enhanced when the instrumentation is used in the multi-frequency mode.

Characterization of nonlinear background components in voltammetry by use of large amplitude periodic perturbations and fourier transform analysis.

Under most experimental conditions, a distinctly nonlinear background current is encountered in all forms of voltammetry which arises from the potential dependence of the capacitance. The nonlinear background current has been successfully modeled under large amplitude sinusoidal ac voltammetric conditions with a fourth order polynomial. The model was applied to a dummy cell containing a nonideal ceramic capacitor and commonly used electrodes. The nonlinearity in behavior of the background capacitance is particularly significant when considering the discrimination between the Faradaic and background contributions in the higher order harmonics resolved in ac voltammetry by Fourier transform-inverse Fourier transform approaches and in the simulation of the background current and hence double-layer capacitance as a function of potential. Typically, measurable background current under large amplitude conditions is detectable in the dc and fundamental to fourth harmonic components in large amplitude ac voltammetry. For analytical purposes, this background current can be corrected on a per harmonic basis without the need for any model. Background correction has been successfully applied to the first four harmonics for the oxidation of ferrocenemonocarboxylic acid over the concentration range of 5-500 microM in aqueous 0.5 M NaCl solution.

Analysis of ramped square-wave voltammetry in the frequency domain

We present a new approach to the analysis of square-wave voltammetry in the frequency domain. By extending our earlier work (J. Electroanal. Chem. 480 (2000) 133) on the numerical simulation of ac sine wave voltammetry, we are able to solve the governing equations when a square waveform of any amplitude is superimposed onto a linearly varying dc potential which is swept at a finite scan rate. By considering the numerical results in the frequency domain by using the fast Fourier transform (FFT) method, we are able to develop a very simple and general form of analysis which will theoretically allow consideration of reaction phenomena over a very wide range of timescales using a single potential sweep. We go on to develop some novel theoretical analyses, which support our numerical results, using an assumption that the applied square-wave signal is superimposed on top of a fixed (or very slowly varying) dc signal. This allows us to give exact and surprisingly simple analytical results relating the amplitude and phase of the output signal at the half-wave potential (at odd multiples of the fundamental frequency), to the amplitude of the applied square-wave signal, for any amplitude of the applied signal. Finally, we give brief experimental results showing qualitative agreement with our simulation results.

A comparison of sinusoidal, square wave, sawtooth, and staircase forms of transient ramped voltammetry when a reversible process is analysed in the frequency domain

Sawtooth and squarewave forms of transient voltammetry have been simulated in order that a detailed comparison with results previously undertaken on sinusoidal and square wave forms of voltammetry could be made in the frequency domain. All theoretical calculations were performed for a reversible process, assuming a linear dc ramped potential is present, and using numerical simulations which were analysed using the fast Fourier transform method. On the basis of this study, the conclusion is reached that transient voltammetric methods, where a periodic signal is superimposed onto a dc potential, are part of a family of techniques having essentially common characteristics, which can all be analysed in a unified theoretical framework, in contrast to current practice of treating them in inherently different manners. The subtle differences that arise are related to the non-linear effects encountered when large amplitudes of the periodic waveform are superimposed onto the dc potential. Experimental data on the oxidation of ferrocene in acetonitrile support the fidelity of the simulations of sawtooth voltammetry.

Retuning the Catalytic Bias and Overpotential of a [NiFe]-Hydrogenase via a Single Amino Acid Exchange at the Electron Entry/Exit Site

The redox chemistry of the electron entry/exit site in Escherichia coli hydrogenase-1 is shown to play a vital role in tuning biocatalysis. Inspired by nature, we generate a HyaA-R193L variant to disrupt a proposed Arg–His cation−π interaction in the secondary coordination sphere of the outermost, “distal”, iron–sulfur cluster. This rewires the enzyme, enhancing the relative rate of H2 production and the thermodynamic efficiency of H2 oxidation catalysis. On the basis of Fourier transformed alternating current voltammetry measurements, we relate these changes in catalysis to a shift in the distal [Fe4S4]2+/1+ redox potential, a previously experimentally inaccessible parameter. Thus, metalloenzyme chemistry is shown to be tuned by the second coordination sphere of an electron transfer site distant from the catalytic center.

A practical approach to applying short time Fourier transform methods in voltammetric investigations

A method for using windowing on a current signal is presented as a means of obtaining amplitude and phase information from electrochemical data transformed from the time domain into the frequency domain. A test method also has been developed to check the reliability of the peak-heights extracted from the data. The limitations have been assessed from both simulations and real experimental data. Additionally, an efficient method for removal of the dc-component in a power-spectrum is presented.

Leveraging e-Science infrastructure for electrochemical research

As in many scientific disciplines, modern chemistry involves a mix of experimentation and computer-supported theory. Historically, these skills have been provided by different groups, and range from traditional ‘wet’ laboratory science to advanced numerical simulation. Increasingly, progress is made by global collaborations, in which new theory may be developed in one part of the world and applied and tested in the laboratory elsewhere. e-Science, or cyber-infrastructure, underpins such collaborations by providing a unified platform for accessing scientific instruments, computers and data archives, and collaboration tools. In this paper we discuss the application of advanced e-Science software tools to electrochemistry research performed in three different laboratories – two at Monash University in Australia and one at the University of Oxford in the UK. We show that software tools that were originally developed for a range of application domains can be applied to electrochemical problems, in particular Fourier voltammetry. Moreover, we show that, by replacing ad-hoc manual processes with e-Science tools, we obtain more accurate solutions automatically.

Systematic evaluation of electrode kinetics and impact of surface heterogeneity for surface-confined proteins using analysis of harmonic components available in sinusoidal large-amplitude Fourier transformed ac voltammetry

A systematic approach to quantifying the electrode kinetics of surface-confined proteins and identifying the impact of surface heterogeneity is presented. The evaluation approach is based on analysis of individual harmonics derived from Fourier transformed large-amplitude ac voltammetry, and their peak current magnitude, I(p)(nomegat) versus frequency, f, relationships. Effectively, variability in the time-scale of each harmonic is expected, and advantage is taken of the fact that each individual harmonic displays a different level of sensitivity with respect to the kinetic evaluation. The data strategy protocols have been examined for the azurin Cu(II)/Cu(I) process when this metalloprotein is immobilized on gold electrodes modified alkanethiols having different chain lengths, using both pure and mixed thiol systems. I(p)(nomegat) versusf relationships also offer the advantage of the ability to detect and allow for the ohmic IR(u) drop effect and allow analyses that are independent of protein surface coverage. Estimation of an electron transfer rate is achievable from this form of analysis. However, experimentally observed waveshapes for each individual harmonic are consistently broader than that deduced theoretically on the basis of their rate constants because of kinetic and/or thermodynamic dispersion. In the mixed thiol systems, and with use of the ac method, kinetic discrimination is achieved for fast processes. This systematic study based on a model protein indicates that a more comprehensive level of evaluation of electrode kinetics can be derived from analysis of the ac harmonics available in large-amplitude ac voltammetry, by initially using I(p)(nomegat)-f data to evaluate the electrode kinetics followed by waveshape analysis to detect heterogeneity effects that give rise to kinetic or thermodynamic dispersion.

Benefits of an FPGA based SRM controller

This paper proposes that a Field Programmable Gate Array (FPGA) based Switched Reluctance Motor (SRM) controller has greater flexibility and potential performance benefits than a typical Digital Signal Processor (DSP) or Microprocessor based system. The benefits stem from the architecture of an FPGA which allows designers to make a completely customisable controller. The architecture also allows enhanced sampling of the phase current and voltages which allows for higher quality data capture which can improve drive performance. A basic overview of the operation of an FPGA is given to provide context of the system architecture. Comparisons between an FPGA and a DSP based system are made within the context of an SRM controller, which further highlight the potential benefits. Lastly a basic concept design of an FPGA SRM based controller is shown and its enhanced phase current capture design methodology explained.

Design procedure for low cost, low mass, direct drive, in-wheel motor drivetrains for electric and hybrid vehicles

Direct drive, in-wheel motors are ideal for electric and hybrid vehicles because the packaging of the drivetrain is so simple, because drivetrain losses are eliminated, and because individual wheel control improves handling and safety. In applications where cost is not a constraint, e.g. solar car racing, direct drive, in-wheel motors are the norm. In-wheel motors are also regularly demonstrated in concept vehicles. However, in-wheel motors are not used for production vehicles because of their high cost and high-unsprung mass. This paper describes a project that addresses these issues through the use of a novel, multiple-airgap, axial-flux, switched-reluctance motor with optimized packaging and low cost electronics. The emphasis of the paper is on how to design the system as a whole.